PLC EXERCISES · DRILL 4 OF 4
Forward reverse motor controlTwo seal-ins and an interlock, in ladder logic.
Build a reversing starter program in the same ControlsTrainer 5000 editor as the levels, then prove only one direction can ever run at a time.
A three-phase motor reverses when two of its supply phases are swapped. A reversing starter does that with two contactors: one wired L1-L2-L3 for forward, the other with two phases crossed for reverse.
If both contactors ever close together, those crossed phases are a dead short. So each direction’s rung has to check that the other one is off. That check is the interlock.
Forward reverse ladder logic
Rung 0: [ Fwd_PB ] in parallel with [ Motor_Fwd ], then [ Stop_PB ], then ]/[ Motor_Rev, then ( Motor_Fwd ).
Rung 1: [ Rev_PB ] in parallel with [ Motor_Rev ], then [ Stop_PB ], then ]/[ Motor_Fwd, then ( Motor_Rev ).
| Contact | Instruction | Why |
|---|---|---|
| Fwd_PB / Rev_PB (normally open) | XIC | Starts that direction while pressed |
| Motor_Fwd / Motor_Rev (the seal) | XIC | Holds its own rung on after the button is released |
| Stop_PB (wired normally closed) | XIC | Its bit is ON until pressed, so the rung opens when Stop is pressed |
| The other direction’s output | XIO | The interlock: true only while the other contactor is off |
Interlocks on a real starter
- Software: the XIO contacts in this program.
- Electrical: each contactor’s normally closed auxiliary (21–22) wired in series with the other contactor’s coil.
- Mechanical: a bar between the two contactors so both can’t physically close.
Reversing a running motor straight away (plugging) slams the load and draws a big current. Most machines need Stop first, and often a short delay, before changing direction.
Common questions
How does forward reverse motor control work?
Two contactors feed the motor. The forward contactor connects L1, L2 and L3 in order; the reverse contactor swaps two of them, which reverses the motor. Each has its own Start button and seal-in, with a shared Stop.
Why do forward and reverse contactors need an interlock?
Because the reverse contactor swaps two phases. If both closed at once, they’d short those phases together. The interlock makes sure one can only close while the other is open.
How do you interlock forward and reverse in ladder logic?
Add an XIO of the other direction’s output to each rung: XIO Motor_Rev on the forward rung and XIO Motor_Fwd on the reverse rung. Each rung is then true only while the other output is off.
Is a PLC interlock enough on a reversing starter?
No. Real reversing starters also use a mechanical interlock and normally closed auxiliary contacts wired into each other’s coil circuits, so a program fault or a welded contact can’t close both.
YOUR NEXT STEP
See it on a real panel
Level 1 wires and runs a motor starter: the same contactor, overload and seal-in rung, with an E-Stop and a restart permission. It’s free.
Training practice. A real reversing starter also needs a mechanical interlock and hardwired auxiliary-contact interlocks, plus the safety functions your risk assessment calls for; a PLC rung alone isn’t a safety circuit.